1. Field of the Invention
The present invention relates to a magneto-resistive effect element, a thin-film magnetic head, a head gimbal assembly, and a hard disk drive, and more particularly, to a magneto-resistive effect element for use in a thin-film magnetic head for a magnetic recording apparatus such as a hard disk drive.
2. Description of the Related Art
Magnetic heads using a GMR (Giant Magneto-resistive) film as a read element have been utilized in a wide range of applications in order to cope with the density of magnetic recording that is increasingly higher. In particular, a GMR element using a spin valve film (hereinafter called an SV film) can provide a magnetic head having a higher sensitivity because of the large change in its resistance to sense current which is applied to the element in order to read recorded data on a recording medium. An SV film consists of stacked films which have a ferromagnetic film whose magnetization direction is fixed in one direction (hereinafter also called a pinned layer), a ferromagnetic film whose magnetization direction varies in response to an external magnetic field generated by a recording medium (hereinafter also called a free layer), and a non-magnetic spacer layer sandwiched therebetween.
Conventionally, a CIP (Current in Plane)—GMR element to which sense current is applied in parallel with film planes has been widely used for a MR element that has an SV film. Recently, a magnetic head which uses a CPP (Current Perpendicular to the Plane)—GMR element to which sense current is applied perpendicular to film planes has been developed in order to cope with a further increase in density. A CPP-GMR element exhibits lower resistance than a TMR (Tunnel Magneto-resistance) element having TMR film, which is an alternative kind of a CPP type element. A CPP-GMR element also exhibits a large output even when it reads data from a track in a narrow width, as compared with a CIP-GMR element. Therefore, the CPP-GMR element is considered to be a highly promising element having a high potential.
However, since the sense current passes perpendicular to the film planes, i.e., boundaries, a CPP-GMR element has the disadvantage that it does not generate sufficient spin dependent scattering on the boundaries, resulting in a small change in the magneto-resistance. Thus, in CPP-GMR elements, the following two measures are generally taken in order to increase the change in the magneto-resistance. One is to increase the number of boundaries to enhance the boundary scattering effect. Since a boundary scattering coefficient depends on a combination of materials which form a boundary, the layer configuration is important. For example, a multi-layer configuration, such as Co/NiFe/Co, in which at least one of two pinned layers has an intermediate layer made of NiFe, is disclosed as an appropriate layer configuration. See, for example, the specification etc. of Japanese Patent Laid-open Publication No. 2003-8103, and No. 2001-52317. The other is to increase the thicknesses of a free layer, a non-magnetic intermediate layer, a pinned layer, etc. in order to increase the scattering of conduction electrons within each layer, i.e., to increase the resistance due to bulk scattering. In this approach, the film thickness is more important than the layer configuration. CPP-GMR elements, indeed, are often made considerably thicker than CIP-GMR elements and TMR elements.
A so-called synthetic pinned layer can be used for the pinned layer of a GMR element. A synthetic pinned layer is a pinned layer that has an outer pinned layer which is a magnetic layer, a non-magnetic metal layer made of Ru or Rh, and an inner pinned layer which is a magnetic layer, stacked in this order, wherein the outer pinned layer and the inner pinned layer are anti-ferromagnetically coupled via the non-magnetic metal layer. In this configuration, the overall SV film is composed of a buffer layer/anti-ferromagnetic layer/outer pinned layer/non-magnetic intermediate layer/inner pinned layer/spacer layer/free layer/cap layer, which are stacked in this order. In a synthetic pinned layer, the outer pinned layer and the inner pinned layer are magnetized anti-parallel to each other, so that the magnetization of the pinned layer is inhibited and stabilized. Further, when the synthetic pinned layer is used in the read element of a head, the shift of a bias point due to a static magnetic field from the pinned layer can be prevented.
While investigations to apply a synthetic pinned layer to a CPP-GMR element have been performed, the film thickness of a pinned layer, i.e., the film thicknesses of an outer pinned layer and an inner pinned layer, tends to be larger by the reason described above.
However, in a CPP-GMR element which uses a synthetic pinned layer, the increase in film thickness of a pinned layer, as described above, creates the disadvantage that it is difficult to ensure the anti-ferromagnetic coupling between the outer pinned layer and the inner pinned layer will occur. This is because the increase in film thickness of the pinned layer causes magnetic moments to be increased in the outer pinned layer and the inner pinned layer. The increase in magnetic moments leads to insufficient degree of strength for the exchange coupling of the non-magnetic intermediate layer, and this means that the anti-ferromagnetic coupling of the outer pinned layer and the inner pinned layer will be weak.
Ideally, in a synthetic pinned layer, an outer pinned layer and an inner pinned layer are configured to be in anti-parallel magnetization directions, and configured to exhibit no effective magnetization as a whole. However, in a thick pinned layer as mentioned above, the range of the effective magnetic field in which the outer pinned layer and the inner pinned layer can be anti-ferromagnetically coupled via the non-magnetic intermediate layer, will be reduced. Further, the temperature at which anti-ferromagnetic coupling of the pinned layer can be maintained under high temperatures will be reduced. Consequently, this leads to an unfavorable situation in which it is difficult to reliably achieve a large change in magneto-resistance in a wider temperature range and in a larger magnetic field.